The present invention is directed to novel apparatus including a cavity transfer mixer for preparing homogeneous mixtures of materials and, more particularly, for commercial preparation of such mixtures, e.g. to the manufacture of rubber-based compositions, adhesive formulations and other mixtures of solid materials.
Cavity transfer mixers are per se old and have been employed in various mixing operations.
In general, they are a form of extruder mixer wherein material is fed into one end and exits through an extruder die at the opposed end. They consist essentially of a hollow cylindrical stator member and a cylindrical rotor member which is rotatable therewithin. The facing cylindrical surfaces on the rotor and stator carry respective pluralities of rows of grooves or cavities positioned so as to cause a mixing as the material traverses the mixer.
British Specification No. 930,339 describes a cavity transfer mixer of this description wherein the grooves are elongate and longitudinally extending. The rows of grooves on each member extend peripherally around the member and are spaced apart axially, the rows on one member being axially offset from the rows on the other member so as to provide an axial overlap of the grooves in adjacent rows on the stator and rotor. Because of this arrangement of overlapping closed cavities on the rotor and stator, material passing through this mixer must travel a path which alternates between rotor and stator cavities. Where a cavity on one member happens to be opposite a land on the other member, the material to be admixed is subjected to simple shear so that it is cut in half before being displaced approximately at right angles to its original direction as it passes into the next cavity.
U.S. Pat. No. 4,419,014 relates to an improved cavity transfer mixer which is particularly efficacious in the practice of the present invention. In accordance with this patent, the rotor and stator cavities are formed as hemispheres arranged in a special configuration. Specifically, these hemispherical cavities are arranged in parallel rows on the rotor and stator such that: (a) the cavities in adjacent rows on the stator are circumferentially offset; (b) the cavities in adjacent rows on the rotor are circumferentially offset; and (c) the rows of cavities on the stator and rotor are axially offset, whereby an overall increase in mixing capacity for the same surface area can be obtained while achieving a desired exponential mixing characteristic in which simple shear mixing is repeatedly interrupted by cutting and turning stages.
The cavity transfer mixer disclosed in the aforementioned U.S. Patent is appreciably more efficient than that described in the British Patent. Specifically, the mixing capacity for the surface area is considerably increased. Moreover, other significant advantages are obtained. The configuration of hemispherical cavities can be arranged so that overlaps occur between three cavities at any given time so that extra mixing or blending is obtained by repeated division of the melt streams. The hemispherical shape of the cavities provides excellent streamlining so that, for example, stagnation will not occur. Other advantages are described in Col. 3.
The aforementioned parent application, Ser. No. 857,692, now U.S. Pat. No. 4,692,352, describes and claims a novel system for incorporating a rubber crosslinking agent in a rubber-based adhesive formulation wherein the crosslinking agent is admixed with the adhesive formulation in a cavity transfer mixer (CTM), e.g. a CTM of the type disclosed in the aforementioned U.S. Pat. No. 4,419,014, thereby providing significant manufacturing advantages. Preferably the crosslinker is incorporated in an oil or plastisizer vehicle. As described in the copending application, a premix of the rubber and other components is first formed in a Banbury in a batch operation. The premix is then transported to the input end of the extruder, to the output end of which the CTM is positioned. The CTM is provided near its leading end with an injection port through which the crosslinking agent is fed. As is discussed, the CTM may be threaded onto the exit end of the extruder or otherwise secured to the extruder. Alternatively, it may be a separately driven, variable speed CTM mounted or secured contiguous with the exit port of the extruder, e.g. by clamping means. In the latter embodiment, the CTM may have a diameter significantly greater than the extruder, thereby providing greater surface area for mixing in the CTM, which in turn allows the addition of larger quantities of additives, i.e. increases output.
Preferably, the process is operated as a continuous one where the premix from the Banbury drops directly into a continuously operating extruder and metered amounts of crosslinker are automatically fed into the CTM with the aid of per se known microprocessors. In other words, since the amount of premix exiting from the Banbury in a given batch time, e.g. 10 minutes is known, as is the rate of passage of the premix through the extruder, the required amount of crosslinker to be admixed can be determined and automatically metered into the CTM as the premix is continuously fed therethrough.
In this manner, employing a CTM in lieu of the 84 inch two-roll mill, a homogeneous adhesive mixture containing the crosslinker is instantaneously obtained and this premix may be immediately conveyed to a calender where the adhesive is applied to a suitable backing material to form an adhesive tape.
The copending application of Elwyn G. Huddleston (instant Applicant) and Richard J. Lacana, Ser. No. 892,677 filed Aug. 1, 1986, now U.S. Pat. No. 4,687,794, describes and claims another process for preparing rubber compositions employing a cavity transfer mixer. As described therein, a CTM is employed to incorporate tackifying resins into rubber compositions, including adhesive formulations.
In the preferred embodiments, as is described in this copending application, all of the components of the adhesive formulation except for the required amount of tackifier are first admixed in an internal mixer, most preferably a Banbury, in per se known manner to provide a substantially homogenous molten premix of the rubber and other components (filler, antioxidant, etc.)
The molten premix is then transported from the Banbury or other internal mixer to the CTM for the addition of the tackifier. Preferably, this is accomplished in a continuous rather than a batch operation wherein the premix is dropped from the internal mixer discharge hopper directly into the input end of a conventional extruder. The CTM is mounted or secured adjacent to the output end of the extruder so that the premix is conveyered to the CTM via the extruder.
In theory, the CTM may be threaded onto the output end of the extruder. Theoretically, the CTM may also be provided with a single port through which the resin is introduced. Both of these designs have utility for purposes of this invention. However, the usefulness of either or both of these constructions is limited from a manufacturing standpoint and consequently they are not preferred, at least with most adhesive systems which are contemplated.
Ser. No. 892,677, now U.S. Pat. No. 4,687,794, further discloses that experiments have shown that when a 6 row CTM was fitted directly with a Davis Standard 16:1 cold feed extruder, a maximum amount of about 5-6 percent resin can be incorporated. While this percentage of tackifier may be entirely adequate for some adhesives, higher amounts of this additive to the premix are required for adhesive formulations generally envisioned.
As is further disclosed in Ser. No. 892,677, now U.S. Pat. No. 4,687,794 the CTM is preferably detached from the extruder, and is independently driven so as to be capable of operating at variable speeds, including speeds appreciably higher than the extruder. In this manner, much greater quantities of resin may be incorporated, i.e. quantities typical of these commonly employed in the contemplated adhesives. It will of course be appreciated that where the CTM is detached and separately driven (as distinguished from being threaded to the extruder), it should nevertheless be mounted in juxtaposition with the extruder (by any per se known mechanical means) so that the molten mass of premix passes directly from the extruder to the CTM.
It is also disclosed to have been found that optimum results may be obtained by increasing the length of the CTM, e.g. to provide a nine row CTM. This may be accomplished simply by providing a single nine row CTM. Alternatively, it may be by means of a variable length CTM, e.g. two or more separate units in sealed relationship to prevent escape of material traveling downstream from one CTM to the next.
By way of illustration, a three row CTM may be fitted directly to a cold feed extruder, e.g. to the output end of a 21/2" extruder. A 6 row variable speed, separately driven CTM may be placed in sealed relationship with the 3 row CTM.
While the CTM may theoretically be provided with a single entry port for feeding the resin, in the preferred embodiment multiple ports are provided along the path (length) of the CTM. By way of illustration, in the embodiment just described employing a 9 Row CTM, excellent results were obtained employing two entry ports for the resin. For example, with a nine row CTM in which the last six rows were driven independently, e.g. at 3-4 times the speed of the main extruder, two injection ports were found to be adequate for the addition of up to 23% hot resin.
Preferably, however, a 9 row unit which is entirely driven independently, will be provided with three spaced ports for injecting the resin, e.g. a first one just before the CTM, a second one at the 3 row position, and a third one at the six row position. It should be however noted, that it will not always be necessary to inject at all three positions. Nevertheless, the use of three ports will provide greater flexibility for the system.